Small advance for a vessel? (7)
I believe the answer is:
minisub
'a vessel?' is the definition.
(thesaurus)
'small advance' is the wordplay.
'small' becomes 'mini' (short for miniature).
'advance' becomes 'sub' (I've seen this in other clues).
'mini'+'sub'='MINISUB'
'for' acts as a link.
(Other definitions for minisub that I've seen before include "small craft" , "exploration vessel" , "Vessel for up to two persons for underwater work" .)